Gasoline engines are a type of internal combustion engine characterized by the fact that the fuel is burned inside the engine. An example of an internal combustion engine is the automobile engine, which works by burning a mixture of gasoline and air in a cylinder containing a piston. This produces heat, which causes the air to expand and force down the piston. The piston turns a crankshaft linked to the wheels.
The amounts of air and fuel mixed in the cylinder is an important parameter in the operating characteristics of the equipment that the engine drives. The air intake is controlled by a throttle, which in the case of an automobile, responds to the driver's operation of an accelerator pedal. Fuel intake is controlled by some sort of fuel valve, which computes a fuel input quantity, F. In the case of an automobile, the fuel input is commonly controlled by a fuel injection valve.
In conventional air/fuel control systems for gasoline engines, a fuel input quantity derived from a basic fuel injection quantity, F.sub.B, is modified by an air/fuel ratio feedback correction coefficient, LAMBDA. Typically, a value for LAMBDA is obtained by sensing the oxygen concentration in the engine exhaust. Other correction coefficients, COEFF, which represent various driving conditions, may also be factored in. Thus, in general, the final fuel input quantity, F, may be expressed as: EQU F=F.sub.B .times.COEFF.times.LAMBDA.
A pulse signal having a pulse width corresponding to the desired fuel input quantity is delivered to the fuel intake valve, which delivers that amount of fuel to the engine.
In many of these conventional air/fuel control systems, the correction coefficient, LAMBDA, is calculated on the basis of proportion-integration-derivative (PID) control. However, a characteristic of PID controllers is that they are designed for linear systems. Some sort of accommodation must be made for nonlinearities, such as is the case with the output of many standard exhaust oxygen sensing devices.
Another approach to air/fuel control is a heuristic approach, which involves using a lookup table to store an appropriate output value for each set of input values. These input values may include the air/fuel ratio as detected in the engine exhaust, as well as other variables. The values entered into the table are chosen by expert knowledge and experimentation. However, a problem with this type of air/fuel control is that it requires substantial memory space for the lookup table. This increases the complexity and cost of the control system.
A need exists for an air/fuel control system that operates well with existing exhaust sensing devices, and that does not have substantial memory storage requirements.